Engines are usually designed with the ability to deliver a peak output, although most engine operation is performed well below this peak value. As such, it can be beneficial to operate with some cylinders inducting air without fuel injection as described in U.S. Pat. No. 6,568,177.
However, the inventors herein have recognized that it can also be advantageous to operate in some conditions with all of the cylinder pumping air without fuel injection, especially in tip-out conditions where the vehicle operator has tipped-out of the vehicle accelerator pedal.
Further, the inventors herein have recognized that when the all of the cylinders are in the fuel shut-off condition, air (still being pumped by the engine) oxidizes the whole catalyst system in the exhaust. This can cause increased NOx emissions when the engine is restarted, since any NOx generated during the cylinder start will not be efficiently converted in an oxidized exhaust system due to the fact that any three way catalyst present generally have poor NOx conversion efficiency in a completely oxidized state or in lean exhaust.
By commencing combustion at a lean air-fuel ratio after operating in the fuel cut operation, it is possible to retain a lean exhaust mixture air-fuel so that any NOx generated can be stored in an emission control device capable of lean NOx storage. Further, lean combustion upon re-enabling combustion provides for greater variety of torque levels, especially when a group of cylinders is re-enabled. In other words, it is possible to provide accurate control of low, or negative, torque levels during engine deceleration conditions, such as during driver tip-out conditions.
In other words, full cylinder cut operation can be done with minimal NOx emissions impact since the NOx trap can operated even in an excessively oxidized state. When the cylinder cut phase is completed, the engine can be operated in a lean-burn mode. The NOx trap retains any NOx from the cylinder re-enablement thereby reducing NOx emissions. Further, the engine can be cycled between lean operation and cylinder cut operation without emissions impact. In addition, the NOx trap, and any three way catalysts in the system, can be regenerated when the NOx trap is filled with NOx or when lean operation is no longer requested. This leads to reduced regenerations of the exhaust system, further improving fuel economy performance.